The present invention relates to the field of anti-counterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authenticating documents and valuable products by luminescent backlit full color images composed of a non-luminescent transmissive color image on one side of a transmissive substrate (recto side) and a luminescent emissive color image on the other side of the transmissive substrate (verso side).
The invented authentication method relies on a device that has a given appearance under normal white light (e.g. daylight, tungsten light, light from a fluorescent tube, etc.) and another appearance or a substantially similar appearance under an excitation light (e.g. UV light).
The present invention is related to see-through devices which also comprise front and back images that form a new image when viewed in transmission. Such devices require a high registration accuracy between front and back images. Prior art see-through devices are present on several bank notes, see book R. van Renesse, Optical Document Security, 3rd edition, Ed. Artech House optoelectronics library, pp. 133-136.
In U.S. patent application Ser. No. 12/519,981, “Data carrier with see-though window and method for producing it”, filed Dec. 5, 2007, inventors Syrjanen et al. propose a data carrier having a see-through portion that allows revealing security features with a different appearance on each side under special lighting conditions. The see-though portion comprises security markings, a developer material and a filtering material both changing the appearance of the security markings The developer material can be luminescent inks and the filtering material UV or IR filters.
In contrast to Syrjanen's invention, the present invention aims at creating full color images visible both under normal light (e.g. daylight, tungsten light, fluorescent light, halogen light) and under an excitation light (e.g. UV light).
In U.S. patent application Ser. No. 12/337,686, “UV fluorescence encoded background images using adaptive halftoning into disjoint sets”, filed Dec. 18, 2008, inventors Zhao et al. propose to create a watermark visible under UV by using UV-active and UV-dull metameric pairs.
In U.S. Pat. No. 4,652,464, “Printing fine art with fluorescent and non-fluorescent colorants”, filed Aug. 5, 1985, inventors Ludlum et al. propose a method combining invisible and visible fluorescent colorants and non-fluorescent colorants for artistic purposes.
In U.S. Pat. No. 6,400,386, “Method of printing a fluorescent image superimposed on a color image”, filed Apr. 12, 2000, inventor No proposes a method for enhancing the visibility of an image in the dark by printing with phosphorescent inks the outline of an original image printed with classical cyan, magenta and yellow inks.
In U.S. patent application Ser. No. 11/666,029, “Color reproduction on translucent or transparent media”, filed Oct. 28, 2004, inventors Perez and Lammens show how to generate a device color profile on translucent or transparent media. They combine reflected and transmitted colors to build lookup tables and profiles for printing. No color prediction model is used.
A further related field is backlit displays for advertising purposes. Such devices use a backlight source illuminating a transmissive color image to achieve bright images that can be seen in the dark, for example in outdoors advertisement. U.S. Pat. No. 6,338,892, “Imageable backlit composite structure”, filed Oct. 13, 1999, inventors McCue et al. claim an image on one side and a light emitting layer formed by phosphorescent or fluorescent materials on the other side. Variations of the light emitting layer for creating authentication elements are not mentioned. No variable intensity or variable color image is formed by the light emitting layer.
In contrast to these prior art inventions, we reproduce, by applying a color prediction model, a luminescent emissive variable intensity or variable color image on one side and a non-luminescent transmissive color halftone image on the other side to obtain a luminescent backlit image formed by the transmission of the luminescent emissive image through the non-luminescent transmissive color halftone image. The verso luminescent emissive image, the recto non-luminescent transmissive image as well as the luminescent backlit image are used for authentication purposes.